Having went through making one out of an old fridge with a plastic interior I can say that you've got a much better start with a metal interior. I had to rip out all the plastic and replace with metal flashing via pop rivets, major pain. What I did was turn the fridge over so the freezer part became the fire box and cut holes from there into the big compartment. I made wooden racks to lay cookie racks on. cut a hole in the bottom (now the top) and put in a hooded chimney with a damper. I use a newer electric hot plate.. all told not a bad smoker but it requires a lot of attention as I the wood tends to catch fire after a while.. so pickup a remote bbq thermometer to keep track of temps. I've also found that I cannot keep temps consistently high enough for anything but smoking fish... with meat you have to be care of botulism when temps are in a certain range.

Surf the web, there are a bunch of DIY smokers sites out there.